(1) Field of the Invention
The present invention relates to a fixing device for fixing an image formed on a recording sheet onto the recording sheet by applying heat and pressure to the recording sheet while the recording sheet passes through a fixing nip formed between a fixing roller and a pressure belt pressed against each other, and relates to an image forming device provided with the fixing device.
(2) Description of the Related Art
An image forming device such as a copier is provided with a fixing device for fixing a toner image formed on a recording sheet, such as a sheet of recording paper or an OHP sheet, onto the recording sheet. In the fixing device, a fixing nip is formed. The fixing device fixes the toner image onto the recording sheet by applying heat and pressure to the recording sheet while the recording sheet is passing through the fixing nip.
In the fixing device, when the recording sheet passes through the fixing nip, the nip pressure applied to the recording sheet may be distributed nonuniformly in the width direction of the recording sheet perpendicular to the recording sheet transport direction, and the transportation speed of the recording sheet may become slower at the ends thereof than at the center in the width direction of the recording sheet. In that case, when the recording sheet enters the fixing nip, stresses directed from the center toward the ends thereof in the width direction of the recording sheet are generated in the initial portion of the recording sheet that has entered the fixing nip. At the same time, in the portion of the recording sheet following the initial portion and having not yet entered the fixing nip, stresses directed from the ends toward the center thereof in the width direction of the recording sheet are generated. This may generate a wrinkle in the recording sheet when the portion of the recording sheet following the initial portion enters the fixing nip.
There have been proposed technologies addressing the above-described problem. For example, Literature 1 (Japanese Patent Application Publication No. H8-137310) discloses a fixing device in which a fixing nip is formed between a heating roller in rotation and a fixedly provided elastic member (made of an elastic material) pressed against the heating roller such that the surface of the elastic member pressed against the heating roller is closer to the heating roller at the center thereof than at the ends thereof, where the center and ends are along the width direction of the recording sheet. With this structure, the load applied to the elastic member becomes greater at the center thereof than at the ends thereof in the width direction of the recording sheet, and the friction resistance between the elastic member and the recording sheet becomes larger at the center. As a result of this, the transportation speed becomes slower at the center than at the ends, making it possible to prevent a wrinkle from being generated in the recording sheet.
Literature 2 (Japanese Patent Application Publication No. 2005-4126) discloses a fixing device which forms a fixing nip between a fixing roller and a pressure belt by causing a pressure pad, composed of an elastic pad and a hard pad, to press the back surface (inner circumferential surface) of the pressure belt so that the pressure belt is brought into contact with the fixing roller, where the center portion of the pressure surface of the elastic pad is shorter than each of the two end portions (the center and ends being along the axis direction of the fixing roller) in the transportation direction of the recording sheet (the circumferential direction).
With this structure, the fixing nip, which is formed with the pressure surface of the elastic pad, becomes longer at the ends thereof than at the center thereof in the transportation direction of the recording sheet, therefore the area of the fixing nip to which the nip pressure is applied becomes longer in the transportation direction at the ends than at the center, and the force to transport the recording sheet becomes greater at the ends of the fixing nip than at the center of the fixing nip, where the center and ends are along the axis direction of the fixing roller, or the width direction of the recording sheet. As a result of this, the transportation speed of the recording sheet becomes faster at the ends than at the center, making it possible to prevent a wrinkle from being generated in the recording sheet.
However, the structure disclosed in Literature 1 has a problem that the heating roller and the elastic member pressed against the heating roller both have a large thermal capacity, and thus a warm-up period increases. Moreover, to increase the friction resistance, the heating roller is pressed by the elastic member, which is provided fixedly, more strongly at the center thereof than at the ends thereof in the width direction of the recording sheet. For this reason, the center of the heating roller in the axis direction of the heating roller may be deformed to be dented depending on the material of the elastic member.
In the case of the above-described structure, when the amount of dent generated at the center of the heating roller becomes larger, the transportation speed of the recording sheet in the fixing nip may become faster at the center thereof (where the moving distance of the recording sheet is longer) than at the ends thereof, where the center and ends are along the axis direction of the heating roller. This results in that the increase in the transportation speed of the recording sheet due to the increase of the amount of dent is larger than the decrease in the transportation speed of the recording sheet due to the increase of the friction resistance. In that case, it is impossible to surely prevent a wrinkle from being generated in the recording sheet passing through the fixing nip.
The fixing device disclosed in Literature 2 uses a pressure belt having a small thermal capacity, making it possible to reduce the warm-up period and restrict the abrasion of the pressure belt. However, the pressure surface of the elastic pad is longer at the ends thereof than at the center thereof in the transportation direction of the recording sheet, where the center and ends are along the axis direction of the fixing roller. Accordingly, since the pressure area of the elastic pad is broader at the ends than at the center, the recording sheet passing through the fixing nip is heated for a longer time at the ends thereof than at the center thereof, where the center and ends are along the width direction of the recording sheet. This may result in that the heat is applied nonuniformly to the recording sheet in the width direction of the recording sheet, causing nonuniform luster or the like due to nonuniform heating, thereby degrading the image quality of the fixed image.